In the mid-twentieth century, in the aftermath of WWII and the Nazi atrocities and in the midst of decolonisation, a new discipline of transcultural psychiatry was being established and institutionalised. This was part and parcel of a global political project in the course of which Western psychiatry attempted to leave behind its colonial legacies and entanglements, and lay the foundation for a more inclusive, egalitarian communication between Western and non-Western concepts of mental illness and healing. In this period, the infrastructure of post-colonial global and transcultural psychiatry was set up, and leading psychiatric figures across the world embarked on identifying, debating and sometimes critiquing the universal psychological characteristics and psychopathological mechanisms supposedly shared among all cultures and civilisations. The article will explore how this psychiatric, social and cultural search for a new definition of ‘common humanity’ was influenced and shaped by the concurrent global rise of social psychiatry. In the early phases of transcultural psychiatry, a large number of psychiatrists were very keen to determine how cultural and social environments shaped the basic traits of human psychology, and ‘psy’ practitioners and anthropologist from all over the world sought to re-define the relationship between culture, race and individual psyche. Most of them worked within the universalist framework, which posited that cultural differences merely formed a veneer of symptoms and expressions while the universal core of mental illness remained the same across all cultures. The article will argue that, even in this context, which explicitly challenged the hierarchical and racist paradigms of colonial psychiatry, the founding generations of transcultural psychiatrists from Western Europe and North America tended to conceive of broader environmental determinants of mental health and pathology in the decolonising world in fairly reductionist terms—focusing almost exclusively on ‘cultural difference’ and cultural, racial and ethnic ‘traditions’, essentialising and reifying them in the process, and failing to establish some common sociological or economic categories of analysis of Western and non-Western ‘mentalities’. On the other hand, it was African and Asian psychiatrists as well as Marxist psychiatrists from Eastern Europe who insisted on applying those broader social psychiatry concepts—such as social class, occupation, socio-economic change, political and group pressures and relations etc.—which were quickly becoming central to mental health research in the West but were largely missing from Western psychiatrists’ engagement with the decolonising world. In this way, some of the leading non-Western psychiatrists relied on social psychiatry to establish the limits of psychiatric universalism, and challenge some of its Eurocentric and essentialising tendencies. Even though they still subscribed to the predominant universalist framework, these practitioners invoked social psychiatry to draw attention to universalism’s internal incoherence, and sought to revise the lingering evolutionary thinking in transcultural psychiatry. They also contributed to re-imagining cross-cultural encounters and exchanges as potentially creative and progressive (whereas early Western transcultural psychiatry primarily viewed the cross-cultural through the prism of pathogenic and traumatic ‘cultural clash’). Therefore, the article will explore the complex politics of the shifting and overlapping definitions of ‘social’ and ‘cultural’ factors in mid-twentieth century transcultural psychiatry, and aims to recover the revolutionary voices of non-Western psychiatrists and their contributions to the global re-drawing of the boundaries of humanity in the second half of the twentieth century.